Developing a balance between excitatory and inhibitory synapses is a critical step in the maturation of functional circuits, and disruption in this process is a key cause of childhood epilepsy. Accumulating evidence suggests that neuronal activity plays a role in achieving this balance in the developing cortex, but the mechanism that controls this process is unknown. In this proposal we plan to use in utero intraventricular electroporation of interference RNA to knockdown expression of the chloride transporter NKCC1 to test the hypothesis that GABA-induced excitation regulates synapse formation in cortical neurons. Our preliminary data show that knockdown of the NKCC1 transporter can shift early GABAergic actions from depolarizing to hyperpolarizing in immature neurons, and as a consequence, excitatory synaptic activity mediated by AMPA receptors is significantly reduced. We plan to explore this effect by testing three main hypotheses: 1) depolarizing GABA current regulates the synaptic integration of newborn neurons in the developing cortex;2) GABA-mediated relief of the voltage-dependent block of NMDA receptor current is responsible;and 3) early GABA excitation is required for the developmental balance between excitation and inhibition in cortical circuits in vivo. Our study proposes to explore a role for GABA in the synaptic integration of newborn cortical neurons, and suggests an activity-dependent mechanism for achieving the balance between excitation and inhibition in the developing cortex. Moreover, as part of our proposal we will examine the ability of bumetanide, a diuretic drug that blocks the NKCC1 transporter, to mimic the NKCC1 knockdown effect and produce long-lasting alterations in cortical circuitry. This particular result may have important clinical implications, since the use of bumetanide has recently been proposed to treat neonatal seizures in humans. Abnormal cortical excitability is associated with a variety of neurological diseases associated with epilepsy, ranging from major cortical malformations to learning disabilities. Understanding the process of circuit formation can help shed light on disease mechanisms and provide safe novel targets for therapeutic intervention. PUBLIC HEALTH RELEVANCE: The balance between excitation and inhibition in the developing and mature cortex is critical for cortical function, and an imbalance can have pathological effects ranging from subtle disorders associated with seizures, to devastating cortical malformations with intractable epilepsy. This proposal examines the intriguing hypothesis that balance is established because early-appearing inhibitory synapses are transiently excitatory, and induce the development of excitatory synapses. Our data is also relevant to planned clinical trials that aim to control seizures in human infants but may inadvertently alter developing cortical circuitry based on the mechanisms that we describe.